A process comprising preheating a material to be treated or processed, heating the material by means of hot vapour such as steam and then cooling the material has been proposed (DE-OS No. 29 43 797) for use in a piece of equipment such as a press or a reaction vessel. That process is operated partly with a heat carrier or exchange agent in liquid form and partly with a heat carrier or exchange agent in vapour form, for example, using water and water vapour, whereby the operating pressure in the installation and thus also the capital investment costs of the installation can be held at a low level. Before that process makes the transition to using vapour as the heat exchange agent, in the last heating stage, the liquid heat exchange agent which is present in the treatment apparatus and in a part of the conduit system is displaced into a liquid storage means, by means of the vapour in the installation. The treatment apparatus is then further heated and held at the high temperature in the heating phase. In that operation, the liquid storage means is disconnected from the treatment apparatus and the vapour which condenses in the treatment apparatus by giving off heat to the material to be heated is passed to a feed water vessel, in the form of condensate, by way of a condensate trap arrangement. From the feed water vessel, the feed water constituted in part by the condensate is recycled to the vapour boiler or like vapour-producing source, for producing the heating vapour. Thereafter, the treatment apparatus is disconnected again from the boiler and cooled in a plurality of steps in succession by increasingly cooler cooling fluid from an array of three liquid storage means, which also include the above-mentioned storage means that previously received the liquid heat exchange agent from the system. That arrangement and mode of operation thus provide for waste heat recovery, with a form of thermal recycling procedure. In spite of the comparatively low level of manufacturing expenditure involved in the installation, the level of heat requirement and thus consumption and at the same time the amount of electrical energy consumed, in comparison with comparable installations without heat recovery, can be reduced to less than half.
However, a disadvantage with the above-indicated process is that the considerable heat recovery effect, by means of the condensate which is passed to the feed water vessel of the boiler or vapour producer, can only be effected with a condensate which does not suffer from contamination or fouling, as such contamination would also contaminate the boiler and thus detrimentally affect operation thereof.